The demand for transportation fuels is expected to increase dramatically in the next 50 years due to the current increases in population as well as improvements in the developing world. Fossil fuels supply more than 70% of current transportation energy consumption due to its ready availability and relatively inexpensive cost. Fossil fuels may temporarily be capable of meeting the future increases in demand, but due to their link to increased concentrations of CO2 in atmosphere, continued use of fossil fuels is expected to result in unpredictable and possibly damaging climate changes. Thus, alternative fuels or energy from clean resources are urgently needed. Among the various possible clean fuel sources, solar stands out as the most promising to meet the terra-Watts level energy gap expected in the near future. Lewis N. S. and G. Nocera Daniel, Proc Natl Acad Sci USA, 2006; 103(43):15729-35.
One difficulty facing solar as a transpiration energy source is its requirement for robust incident sunlight. Methods of storing solar energy in an efficient manner are currently lacking. Splitting water using solar energy represents the ideal pathway to store and utilize solar energy as transportation fuel. Technical approaches to solar water splitting either fall into the category of thermal or thermochemical cycles, or follow the photoelectrochemcial routes. Both approaches are under rapid development, with much study into new catalysts for the oxidation half of the water splitting process.
In native photosynthesis, water oxidation is facilitated by Mn-containing ligands at the oxygen evolving center of PSII. Though highly efficient, the ligands are unstable under the extremely oxidative conditions of artificial systems, and need to be re-generated quickly to maintain the continuous operation of photosynthesis. Also, in artificial photosynthetic systems in situ re-generation of catalysts is almost impossible. The RuO2 and IrO2 catalysts are among the best known of prior catalysts for these artificial systems. These complexes, however, are inappropriate for use on a large scale due to the low natural abundance of the materials. Therefore, there is a need for efficient, stable and readily available catalysts for water oxidation and processes using these catalysts for efficient water oxidation reactions.